Touch screens are present in many different types of common modern electronic devices, such as smartphones, tablets, smartwatches, wearables, laptop computers, and so on. In these electronic devices the touch screen serves as an output device as well as an input device. The touch screen typically includes display layer and a sensing layer. A touch screen controller is coupled to the sensing layer and operates to process signals therefrom to detect inputs by a user. The sensing layer includes touch sensors, and is attached to or formed as an integral part of the display layer, which may be a LCD, IPS, or AMOLED display, for example.
A typical sensing layer of a touch screen is a capacitive sensor array including a number of force or drive lines and orthogonally arranged sense lines. These lines are made from suitable conductive materials, the drive lines are formed on one sub-layer of the sensing layer and the sense lines formed on another sub-layer, with these sub-layers being separated by a transparent insulating material such as an insulating dielectric layer. The overlap of the drive lines and the orthogonally arranged sense lines with the insulating material between forms an array of capacitive sensors. In operation, a drive signal, which is typically a periodic waveform such as a pulse train, is applied successively to the drive lines. As the drive signal is applied to a given drive line, the capacitive coupling between that drive line and the sense lines results in capacitive coupling of the drive signal to the sense lines to thereby generate sense signals on the sense lines responsive to the drive signal.
The value of the sense signal generated on each sense line is a function of the capacitive coupling between that sense line and the drive line receiving the drive signal. This capacitive coupling changes in response to a user's finger, or other touch device such as a stylus, being proximate the sensor nodes formed at the overlap of the drive and sense lines. This change in capacitive coupling of the drive signal to the sense lines will result in a change in the sense signal generated on the sense lines, and in this way the sense signals indicate whether a user's finger or other touch device is adjacent a given sensor node in the touch panel.
In operation, a user touches a surface of the touch panel or hovers his or her finger (or a suitable object such as a stylus) above the touch panel, and the capacitive sensors generate corresponding electronic sensor signals that are provided to the touch screen controller. From these sensor signals, the touch screen controller determines touch data values, from the data values determines the type of touch event or hover event input by the user to the touch screen, and then provides this information to processing circuitry, such as a system on a chip, in the electronic device.
Conventional sensing techniques work adequately in the absence of hardware defects. However, sometimes there may be a hardware defect with one or more capacitive sensors that arises during manufacture, or sometimes after manufacture, for example as a result of stresses undergone by the device. In such situations, conventional sensing techniques may provide inaccurate results. Thus, new sensing techniques and/or methods of compensation are needed so as to be able to provide for suitable operation even in the presence of hardware defects.